In the second edition of Bergey, the Gram-positive bacteria are divided into two large phyla, the Firmicutes and the Actinobacteria. Some 2500 species are known, but a substantial proportion of these belong to just a handful of genera. Gram-positive bacteria mostly have a chemoheterotrophic mode of nutrition and include among their number several important human pathogens, as well as industrially significant forms.
The base composition of an organism's DNA can be expressed as the percentage of cytosine and guanine residues (per cent GC content); the technique is used widely in microbial taxonomy, and the Gram-positive bacteria are divided into those whose GC content is significantly over or under 50 per cent. It is convenient to consider groupings within the high GC and low GC forms as follows:
Phylum Firmicutes (low GC): spore-forming non-spore forming mycoplasma
Phylum Actinobacteria (high GC): actinomycetes coryneform bacteria
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